Conventional open-loop hydraulic drilling systems manage bottomhole pressure (“BHP”) by adjusting the equivalent circulating density (“ECD”) of the fluids, sometimes referred to as mud, disposed within the wellbore. The ECD is the effective fluid density exerted by a circulating fluid against the formation that takes into account the circulating frictional pressure on the fluids returning to the surface and is a function of the injection rate of the mud pumps, the properties of the injected fluids, and the true vertical depth of the wellbore. Under static conditions, when circulation is suspended, circulating frictional pressure is lost and the BHP tends to drop. In narrow pressure windows, this drop may cause the BHP to fall below the pore pressure, potentially inducing a kick, or unintentional influx of formation fluids, into the wellbore. In such circumstances, to prevent a kick, heavier mud weight fluids may be used to maintain the BHP at a pressure higher than that of the pore pressure of the formation. As such, the driller must pay careful attention to the ECD and pressure profile of the wellbore during all drilling operations including drilling, making connections, and tripping into and out of the hole.
In contrast, closed-loop hydraulic drilling systems manage BHP by adjusting the choke settings of a choke manifold, typically disposed on a platform of the floating drilling rig, as part of a pressurized fluid return system. A rotating control device, active control device, or other annular sealing system seals the annulus surrounding the drill string or drill pipe and returning fluids are diverted to the choke manifold. Because the annulus is sealed pressure tight, surface backpressure may be applied and controlled by adjusting the choke settings of the choke manifold. Under static conditions, when drilling ceases, surface backpressure may be provided by the choke manifold, instead of using fluids with heavier mud weights, to maintain the BHP above the pore pressure of the formation. In addition to preventing kicks, and alleviating a number of pressure related drilling problems, the closed annular system with pressurized fluid returns, sometimes referred to generally as a managed pressure drilling (“MPD”) system, allows for the accurate control of annular pressure during all drilling operations including drilling, making connections, and tripping into and out of the hole, as well as completions.